The present invention relates generally to textile yarn sensors and, more particularly, to a yarn sensor having an electronic component housing made of plastic with a measuring gap for monitoring at least one parameter of a running yarn.
Yarn sensors of diverse types are employed at work stations of textile machines, for example of spinning or bobbin-winding machines. Metallic housings, in which the electronic components have been installed, can be used for yarn sensors. Such metallic housings are customarily mass-produced by diecasting, since the diecasting process is considerably more cost-effective compared with a cutting operation, for example. Metals that are suitable for diecasting are aluminum, zinc or magnesium. Although aluminum is advantageous as being lightweight, it has the disadvantage that no fine contours can be formed from this material by means of a diecasting process. Although zinc permits the formation of fine contours because of its low viscosity, it is twice as heavy as aluminum and is very brittle. Fine contours can be formed with magnesium, and this material is approximately as light as aluminum, but it is not resistant to reaction with air and therefore is subject to oxidation.
If plastic is selected as the housing material, the above mentioned disadvantages can be avoided. The advantages of low material costs, low production costs, low weight, as well as resistance to chemicals can be used. The formation of fine contours is possible without problems. However, a plastic material acting as an insulator does encourage electrostatic charging, which can be generated if an electrostatic charge created by friction is passed to the yarn sensor by the traveling yarn. In case of capacitive measurements, undesirable measuring errors can occur in this way.
Such electrostatic charging is to be prevented by means of a yarn measuring device in accordance with U.S. Pat. No. 3,377,852, for example, whose block-shaped body is made of an insulating material. Electrodes are arranged on the block on both sides of the measuring gap such that they constitute a capacitor, which is suitable for yarn measuring purposes. The surface in the measuring gap, including the surface of the electrodes arranged therein, is coated with a thin layer of material which acts as a weakly insulating material. With the aid of this layer, it is intended to distribute electrical charges in the measuring gap and to slowly dissipate them. The insulating effect, or the electrical conductivity of the layer, is intended to be such that in no case does it act as an extension of the electrode surface, since this could lead to the distortion of the measurement results. It is specifically pointed out in the publication that the required function of the measuring device is no longer provided if the electrical resistance of the layer is set too low.
German Patent Publication DE 39 29 895 A1 describes a yarn break detector with a metallized plastic housing. The front of the housing is not metallized, so that the shielding remains incomplete and interfering effects can act from the outside. This yarn break detector makes it possible to detect whether or not there is yarn within the detection range. This yarn break detector cannot meet increased requirements made on the quality of measurements.
European Patent Publication EP 0 401 600 A2, discloses a measuring head having a housing extruded from plastic for measuring, or monitoring, parameters of a running yarn. An optical and a capacitive measuring element are combined inside the measuring head and are both arranged in the housing together. A support plate with the components for optical and capacitive measuring has been inserted into the housing and screwed together with it. Toward the measuring gap, the light source and the light receptor are each covered by a disk. The capacitor plates for capacitive measurement are applied to or embedded in the disks in the form of an electrically conducting transparent layer of metal.
However, these known yarn sensors are not capable of removing or avoiding a number of undesirable effects. For example, in measuring elements or processors, such as are arranged in the interior of yarn sensors, or of the housings, waste heat is often generated, which results in a considerable increase of the interior housing temperature. Increased temperatures can result in undesired and disadvantageous so-called component drifting in the electrical components installed in the housing. Magnetic, or electrical fields, in particular fields with oscillating field strengths, which originate in the vicinity of the yarn sensor, can distort the measured result in an undesired manner.